Be aware that no current AVR is guaranteed to work with 4K... 4K will need new HDMI standards to be in effect and while all those AVRs (including Onkyo, Sony, etc) claim they support 4K, they do not have the FULL standard HDMI protocol, they only support a subset of 4K which means some of the higher bandwidth 4K playback will not work.
When 4K players come out, they will need the updated HDMI standards be in place to work. The reason why no one has it is because the HDMI chips for the new standards have not been designed and made until the specs are finalized.
If I were you, I'd wait until the 4K standards are finalized and implemented and are proven to work.
BTW, this 4K limitation will also apply to the current 4K TV sets. The only sets that MAY work will be the Samsung 4K sets with the one connector box. The box is supposed to contain the upgraded HDMI hardware so that owners will be able to use it with future 4K standards. Sony has already admitted that their current 4K sets will prossibly not work with 4K players down the road...
Oh yeah, between Yamaha and Pioneer, I like Pioneers digital amps. They can handle low impedance speakers where a lot of regular amp designs cannot... But admittedly, some people do not care for the sound from digital amps..